Near edge X-ray absorption spectra of some small polyatomic molecules

We report high resolution measurements of near edge X-ray absorption fine structure spectra (NEXAFS) at the carbon and oxygen K edges of the series of related organic molecules acetaldehyde, acetone, formic acid, methanol, dimethyl ether, and paraldehyde. The spectra are compared with previous measurements of these gases and formaldehyde and with theoretical calculations. Much more fine structure is resolved than previously, particularly at the carbon edge. The results are in good overall agreement with the theoretical predictions of Plashkevych et al. (Chem. Phys. 2000, 260, 11) that the methyl group spectrum is relatively independent of the functional groups to which it is bonded, as are the carbon and oxygen carbonyl group spectra. On the other hand, oxygen atoms in hydroxyl and ether groups are strongly influenced by neighboring atoms. All of the carbon edge spectra investigated show fine structure, and only in the case of acetone do the density of states and number of vibrational degrees of freedom begin to obscure fine detail. The term values of the strongest transitions to states of acetaldehyde and acetone at the C 1s and O 1s edges are in reasonable agreement with theoretical calculations. The splittings of the C 1s(-1) 3p(CH3) and 3p(CO) Rydberg states due to the low symmetry of the molecules are 0.6-0.7 and 0.38 eV for both molecules, in good agreement with theory. Absolute values of the photoabsorption cross-section, where available in the literature, show moderate to very poor agreement with measured values.